1. Field of the Invention
This invention relates to diaphragm devices for cameras using electric or magnetic optical elements whose transmittance is changed by applying an electrical field or magnetic field, or so-called solid-state diaphragm devices, and more particularly to an electrode arrangement of the liquid crystal cell in the solid-state diaphragm device. 2. Description of the Prior Art
Diaphragm devices for cameras have, in the past, been mainly mechanical. However, a mechanical diaphragm necessitates a control mechanism therefor, and in the case of cameras using interchangeable lenses, there is also a need for a complicated translation mechanism between the diaphragm mechanism in the interchangeable lens mounting and the control mechanism in the camera body. This raises the cost of the camera, the bulk and size, limits the degree of design freedom, and causes other disadvantages.
With this in mind, proposals have been made to use an electrical or magnetic optical element of which the light transmittance is changed by applying an electrical field or magnetic field as the diaphragm in the camera and to provide a variety of so-called solid-state diaphragms. The above-described electrical or magnetic optical element may be in the form of a Kerr cell, transparent ferroelectric material (PLZT), liquid crystal, or Faraday element in combination with polarizers or analyzers. Elements utilizing electrophoresis or electrochromy have also been suggested.
These solid-state diaphragms have many characteristic features which the conventional mechanical diaphragms lack. First of all, no mechanical transmission mechanism is necessary, thus facilitating a minimization of the size and weight. This leads to a significant increase in the degree of freedom in camera construction. Because the diaphragm operates without shock and silently, an unpleasant impression is not given to the operator. Again, the provision of one switch is sufficient to control the closing down of the diaphragm once the depth of field is recognized. Also, what is necessary for the solid-state diaphragm to function is an electrical circuit, and this matches the recent trend of increasing the use of electronic elements in cameras, and can constitute a great step towards the achievement of a completely electronized camera in the future.
However, a solid-state diaphragm device using a liquid crystal cell has the following drawbacks.
In an area control type solid-state diaphragm device, when the precision of aperture control is made, for example, 1/3 stop in the Apex system, the provision of a 7-stop range of variation of the aperture size requires the selection of 3.times.7=21 different aperture values. On condition that the drive is static, therefore, almost the same number of electrodes must be employed in the liquid crystal cell. Further since, in single lens reflex cameras, the precision of aperture control is increased to 1/8 unit. When this is realized in a solid-state diaphragm device, whose aperture varies in 7 stops, it is necessary to estabish 8.times.7=56 distinctly different aperture values. Therefore, an even greater number of electrodes must be employed. In practice, however, it is very difficult to carry out the assembly of so great a number of electrodes in connection with a drive circuit therefor.
On the other hand, the use of a matrix driving system makes it possible to reduce this number of electrodes. But note that the mere substitution of this matrix driving system for the driving system does not suffice to actually solve the problem. In more detail, where the multiplicity of electrodes to be driven by the matrix driving system are configured to annular shapes of different area and arranged in concentric relation to each other, the power source lines connecting the respective individual electrodes in the multiplicity to external electrical circuitry must be provided beneath the other transparent annular electrodes positioned outwardly of themselves. Thus, there is a need to employ a great number of strips of an electrically insulating layer between the electrodes and the lines. Moreover, this gives rise to an alternative problem which makes the structure of the liquid cell very complicated.